ES2246042T3 - MEMORY CELL - Google Patents

Abstract

In a ferroelectret or electret memory cell a polymeric memory material is a blend of two or more polymeric materials, the polymeric material being provided contacting first and second electrodes. Each electrode is a composite multilayer having a first highly conducting layer and a conducting polymer layer, the latter forming a contact between the former and the memory material.

Description

Memory cell.

The present invention relates to a cell of memory comprising a polymeric memory material with ferroelectric or electret properties and capable of being polarized and of showing hysteresis and in which polymeric memory material it is provided in contact with first and second electrodes, and in the that the polymeric material is a mixture of at least one first and one second polymeric material, said first polymeric material being a ferroelectric or electret polymeric material. The present invention also relates to a process for conditioning before use of a memory cell comprising a polymeric memory material with ferroelectric properties or of electret and able to be polarized and to show hysteresis and in which the polymeric memory material is provided in contact with first and second electrodes, and in which the polymeric material it is a mixture of at least a first and a second material polymeric, said first polymeric material being a material ferroelectric or electret polymer.

In particular, the present invention relates to storage and / or data processing devices based on structures in which thin films of ferroelectric or electret are subjected to electric fields configured by electrodes that They are part of the structure.

In recent years, they have been shown non-volatile data storage devices in which each information bit is stored as a polarization state in a localized volume element of an electrically material polarizable Non volatility is achieved because the material can conserve its polarization even in the absence of electric fields taxes externally. Until then, polarizable materials they have normally been ferroelectric ceramic materials, and the writing, reading and deleting data has involved the application of electric fields to the ferroelectric material in localized cells in memory devices, causing the material in a given cell toggle or not toggle its polarization direction, Depending on your previous electrical history.

During normal operation of the device in question, the ferroelectric can undergo a prolonged or repeated electrical intensity and / or numerous polarization reversals. This may cause the ferroelectric to suffer fatigue, i.e., deterioration of the electrical response characteristics required for normal operation of the device. Thus, the ferroelectric material can exhibit reduced remaining polarization that produces reduced switching current signals under induced polarization inversion. In addition, the fatigue process is sometimes accompanied by an increased coercive field that makes it more difficult for the device to switch from one polarization direction to another and slows the switching procedure.

Another undesirable phenomenon of aging is the development of a footprint ; that is, if a ferroelectric memory cell is left in a given polarization state for a period of time, it can become increasingly difficult to reverse the polarization direction, and an asymmetry develops in the fields required to switch polarization in any address.

The footprint can also aggravate problems related to another phenomenon that occurs in the passive matrix addressed memory devices in particular, namely the disturbance : this corresponds to a change in the state of polarization of the ferroelectric, usually the loss of polarization or even the appearance of polarization inversion, when the ferroelectric is unintentionally subjected to repeated or prolonged electric fields of magnitude less than the coercive field. Such disturbing fields may arise in non-addressed memory cells as a side effect of the normal operation of the device. An example is the exposure of voltage V_ {s} / 3 in unaddressed cells in a passive array during write operations using a write voltage V_ {s} in unaddressed cells (see, for example, discussion of protocols for impulse in the Norwegian patent application nº 20.003.508, belonging to the present applicant).

The resolution of the problems listed above is essential for a successful commercialization of ferroelectric-based devices as set forth herein. A lot of effort has been invested in these issues, such as those referring to devices that use inorganic ferroelectrics. The latter are essentially based on two families of perovskites, i.e., lead zirconate titanate (PZT) and layered perovskites such as strontium and bismuth astalate (SBT) and lanthanum modified bismuth titanate (BLT). Among these, SBT and BLT show good fatigue resistance in condenser-type memory cell structures with metal electrodes such as Pt. However, the polarization, electrical leakage and switching characteristics are lower compared to PZT, in addition to being They require high temperatures during manufacturing. On the other hand, initial attempts to use PZT in conjunction with metal electrodes were unsuccessful for most memory applications due to rapid deterioration during repeated switching. As a result of intensive research efforts, it was shown that the integral constituents of the ferroelectric material, such as oxygen in PZT, are lost during the switching of the ferroelectric, leading to holes that migrate to the electrodes and creating interlocking sites that inhibit the domain switching and carry fatigue in the devices. A strategy that has been proven successful in counteracting this phenomenon is the use of conductive oxide electrodes, preferably with a grid structure equal to or similar to that of the ferroelectric in volume, which neutralizes the oxygen gaps that reach the electrode / ferroelectric interface. Some examples of candidates for electrode materials in the case of oxide ferroelectrics such as PZT are RuO 2, SrRuO 2, indium titanium oxide (ITO), LaNiCO 3, lanthanum and strontium cobaltate (LSCO) and yttrium, barium and copper oxide (YBCO). An alternative to the strategy referred to above to provide a supply of critical atomic species in the electrodes is to insert sinks for holes in the ferroelectric in volume by doping and / or stoichiometry adjustment. This approach has been used in PZT by introducing donor dopants such as Nb and La, which substitute at the Zr or Ti sites and neutralize oxygen gaps.

Other improvements and adaptations to different inorganic ferroelectric compositions have emerged, constituting a large body of prior art relating to inorganic ferroelectric films and, in particular, ceramic. For more background information on the prior art, the reader is referred, for example, to: SB Desu "Minimization of Fatigue in Ferroelectric Films", Phys. Stat. Sol. (A) 151, 467-480 (1995); K.-S. Liu and T.-F. Tseng: "Improvement of (Pb_ {1-x} La_ {x}) (Zr_ {y} Ti_ {1-y}) _ {1-x / 4} O_ {3} ferroelectric thin films by use of SrRuO_ {3 } / Ru / Pt / Ti bottom electrodes ", Appl. Phys. Lett. 72 1182-1184 (1998), and: S. Aggarwal et al .: "Switching properties of Pb (Nb, Zr, Ti) O_ {3} capacitors using SrRuO_ {3} electrodes", Appl. Phys. Lett. 75 1787-1789 (1999). As will be explained below, however, the authors of the present invention do not know of any prior relevant techniques in the present context of fatigue reduction in devices that use organic or polymeric electrets or ferroelectrics .

As described in patent applications submitted by the present applicant for materials organic-based ferroelectric and, in particular, polymeric can provide very considerable advantages for use in devices of memory and / or processing compared to their counterparts inorganic However, fatigue and footprint problems have been also found in polymer-based ferroelectrics. If I dont know solve these problems, this will significantly reduce the commercial potential of the devices based on said materials. Unfortunately, the remedies that have been developed for counteract fatigue in inorganic ferroelectric can not provide help in this case, due to fundamental differences in chemistry and basic ferroelectric properties (for example, moving dipoles versus permanent dipoles).

Thus, there is a need for strategies and remedies that can combat the phenomena of fatigue, footprint and disturbance in memory and / or processing devices based on organic electrets or ferroelectrics and, in particular, polymeric

According to the above, a main object of the present invention is to provide basic strategies for avoid or reduce the harmful effects of field strength electrical in organic ferroelectric or electret materials, in particular polymeric, used in devices for data storage and / or processing.

A further object of the present invention is provide explicit descriptions of cell structures of memory in which the entry into operation is prevented or delayed of the basic mechanisms of fatigue, footprint and disturbance by commutation.

One more main object of the present invention is to list particular classes of materials for incorporation into structures of devices resistant to fatigue, footprint and disturbance and offer lists of preferred embodiments of particular relevance

The objects mentioned above, as well as the characteristics and advantages are achieved with a cell of memory according to the present invention, characterized in that each electrode is a composite multilayer electrode comprising a first layer of highly conductive material and a second layer of conductive polymer, the conductive polymer forming a layer of contact between the highly conductive material and the material of memory.

Preferably, the first polymeric material is a copolymer and preferably the second polymeric material is a homopolymer, while preferably the material highly conductor can be a metallic material.

Preferably, a barrier layer is provided between the first and second electrode layers and, in the case of that highly conductive material is a metallic material, the layer barrier is an oxide, nitride or boride of said material metal.

In a first preferred embodiment of the memory cell according to the invention, the memory material is provides sandwich between electrodes.

In a second preferred embodiment of the present invention, the electrodes are provided on surfaces opposite a bridge of insulating material, extending the first electrode beyond the insulating bridge, and the memory material is provides on exposed surfaces of the electrodes first and second and extends between the electrodes on the surfaces sides of the insulating bridge.

In the preferred embodiments of the memory cell according to the invention is advantageous that the material of memory and electrodes are provided as layers of film fine, so that the memory cell constitutes a structure of thin film in layers.

The aforementioned objects, as well as additional features and advantages are also achieved with a procedure for conditioning before using a cell memory according to the invention characterized by application to electrodes of a positive pulse voltage pulse train and alternate negatives that generate an electric field capable of polarizing the memory material in any direction, and subjecting the memory material to a series of successive investments of the polarization direction thereof.

Finally, a memory cell is used according to the invention in a ferroelectric or electret memory device of passive or active addressable matrix.

The invention will be better understood from the following detailed description of various embodiments preferred and with reference to the figures of the drawings, in the that

fig. 1 shows a cross section schematic of a first embodiment of a cell of generic memory according to the prior art,

fig. 2 shows a second section schematic cross-section of a second embodiment of a generic memory cell according to the prior art,

fig. 3 shows hysteresis curves for a ferroelectric material before and after fatigue,

fig. 4 shows a cross section schematic of a first preferred embodiment of the memory cell according to the invention, and corresponding structurally to the generic embodiment of fig. one,

fig. 5 shows a cross section schematic of a second preferred embodiment of the memory cell according to the invention and corresponding structurally to the generic embodiment of fig. one,

fig. 6 shows a cross section schematic of a third preferred embodiment of the memory cell according to the invention and corresponding structurally to the generic embodiment of fig. 2.

Now follow a brief exposition in several ways Generic Embodiments of the Prior Art of Memory Cells as background and introduction to the present invention that is based in its general aspects in similar memory cells structurally

Fig. 1 shows an embodiment Conventional generic memory cell 1 technique previous. The memory cell 1 comprises a memory material 2 such as a ferroelectric or electret material inorganic or organic and in the latter case preferably a polymer, sandwich between a first electrode 3 and a second electrode 4. This is a purely passive memory cell, but can be connected to a switching transistor to form a active memory cell. The latter can be of the consistent type in a transistor and a memory cell and called a circuit of 1T-1C memory or it may consist of more than one transistor and a memory cell, and so on, forming by example 2T-2C memory circuits, etc. The material of memory 2 is polarized by an electric field generated when voltage is applied to electrodes 3, 4.

Fig. 2 shows another embodiment generic of a memory cell 1 of the prior art. In the present specification, the first and second electrodes 3, 4 they are mutually isolated by a bridge of insulating material 5, while the first electrode 3 extends somewhat beyond the insulating bridge 5. Memory material 4 is then provided on the exposed surface of electrodes 3, 4 and covers both these as the side surfaces of the insulating bridge 5. In this case, memory material 2 is polarized in the region that extends between the first and second electrodes 3, 4 in proximity with and adjacent to the side surfaces of the insulating bridge 5. Also in this case the electric dispersion field will be generated when voltage is applied to electrodes 3, 4, which causes the polarization of memory material 2.

Fig. 3 shows the hysteresis curves for the polarization of a ferroelectric or electret material versus to an electric field E applied. The polarization P generated can be positive (+ P_ {r}) or negative (-P_ {r}) according to the polarity of the electric field applied. The first loop of hysteresis I denotes the hysteresis curve of a material ferroelectric or electret that has not been subjected to fatigue, is that is, the reduction in polarization achievable after a large number of switching cycles. After having undergone the fatigue, the ferroelectric or electret material may show a hysteresis loop similar to the second hysteresis II curve and it you will see that the attainable polarization of the material has been reduced considerably when compared to the hysteresis I curve. fig. 3, + E_ {c} and -E_ {c} respectively denote the field Electrical coercive positive and negative. However, you must Note that E_ {c} does not have to be the same for the curves of hysteresis I and II; actually, the coercive field E_ {c} is something higher in the hysteresis II curve.

Now a first form of preferred embodiment of the memory cell according to the present invention in some more detail with reference to fig. 4. By convenience, the memory cell 1 shown in fig. 4 se will contemplate as ferroelectric, that is, memory material 2 shows a ferroelectric behavior, although it could be a electret material equally capable of being polarized and displaying hysteresis. Layer 2 of memory material is provided in sandwich between the first and second electrodes 3 and 4 that are in contact with the memory material on both sides of it. How it will be seen in fig. 4, electrodes 3, 4 are provided as bilayer electrodes. The layer 3a in the electrode 3 is a material highly conductive, for example, a metallic material, and the second layer 3b is a conductive polymer that, for example, could be PEDOT, PANI, polypyrrole or other polymers with conductive properties. He second electrode 4 similarly comprises a first layer of a highly conductive material 4a which can be a metallic material e also a second layer 4b of conductive polymer. Should it is understood that the materials in both electrodes will be preferably similar and that as a highly conductive material a metallic conductor may be preferable, for example of titanium or aluminum

Regarding memory material 2, you must Note that to optimize the ferroelectric memory material based on polymers, it has been proposed to mix a polymer ferroelectric with other polymers, which do not necessarily show a ferroelectric behavior. A mixture has also been tried of a strongly polarizable polymer and a weaker polymer polarizable In addition, at least since 1980 it has been known in the technique the use of several copolymers. In this regard, it can be done reference to a German DE-OS application pending 36 02 887 A1 assigned to Bayer AG, which reveals the use of Static or dynamic RAM ferroelectric capacitors (SRAM or DRAM). In addition to inorganic ferroelectric materials, this publication also proposes organic ferroelectric materials as polymers with easily polarizable atoms and in that case specifically polyolefins with fluorine atoms and similar to poly (vinylidene difluoride), or polymers with groups strongly polarizable terminals such as vinylidene polycyanide (PVCN). The desired optimization of these memory materials can take place using copolymers such as PVDF-TrFE, or mixtures, for example, with PMMA polymethylmethacrylate, or PVCN copolymers with polyvinyl acetate.

The memory material 2 thus provided in sandwich between electrodes 3, 4 is a mixture in a proportion specific to a first polymeric material 2a, which in this case of Course is a ferroelectric polymer. More particularly, the material 2a may be a copolymer, for example PVDF-TrFE, while a second polymeric material 2b is represented in the appears as islets in memory material 2. This second polymeric material 2b that forms the mixture is preferably a homopolymer, for example PVDF.

In fig. 5 a second form of preferred embodiment of memory cell 1 according to the invention, in which the same characteristics are denoted with the same reference numbers in the embodiment of fig. 3 and in all aspects it is similar in terms of materials and structures Therefore, an exhibition will not be offered in this case of the last. The main difference between the embodiment of fig. 4 and the embodiment of fig. 5 is that it has provided a barrier layer 3c; 4c at electrodes 3; 4 between layers 3a; 4a of a highly conductive material and layers 3a; 3b of a conductive polymer. The main function of the barrier layer is to prevent any undesirable reaction between the material highly layer 3a conductor; 4a and the conductive polymer of layers 3b; 4b If the highly conductive material of layers 3a, 4a is a metallic material, for example aluminum or titanium, the barrier layer could be provided as an oxide, nitride or boride thereof and be formed in a procedure stage immediately after deposit of the metallic material subjecting the latter to a treatment of oxidation, nitriding or erasure, as you will understand well experts in the art.

A third preferred embodiment of the memory cell according to the invention is shown in section schematic cross section in fig. 6 and corresponds to the form of generic embodiment of fig. 2. In this report descriptive, memory material 2 is not provided in sandwich, but above electrodes 3, 4. The first electrode 3 is similar to the electrode shown in the second embodiment of fig. 5, that is, formed with three layers 3a, 3b, 3c as is the case with the embodiment in fig. 5. A bridge of insulating material 5 on top of the first electrode 3 so that the last extend in some way beyond the insulating bridge 5 with the surface of the exposed conductive polymer layer 3b. He second electrode 4 is then provided in the insulating bridge 5 and is in all respects structurally similar to the first electrode 3. The first layer 4a of highly conductive material 4 it is covered by the barrier layer 4c and above the layer 4b of conductive polymer is provided with its outer surface exposed. Then, the memory material 2a consisting of the mixture of two polymeric materials 2a, 2b that for all purposes practical are similar to those mentioned above are provides on the electrodes in the manner represented in the fig. 6c, that is, by contacting the polymer layer conductor 3b, 4b of electrodes 3, 4. Memory material 2 is extends over the side edge of the insulating bridge 5. When applying a voltage to the electrodes the polarizing field generated will be then a dispersion field that extends over the side of the insulating bridge 5 between the electrodes. Depending on the geometric proportions involved in the characteristics structural aspects of the embodiment of fig. 6, the field polarizer could also be generated as an inclined lateral field extending between electrodes 3, 4. In any case, it will form a polarized region in memory material 2 in the lateral edges of the bridged electrode arrangement of the memory cell 1. The advantage of the embodiment of fig. 6 is in particular that the memory material 2 comprising the mixture of two polymeric materials 2a, 2b can be arranged in one final stage, thus avoiding its submission to chemical regimes or thermal not compatible in the procedural steps to provide the electrodes 2,3.

In the manufacturing process, the cell of memory 1 will be subjected to heat treatment, usually at quite low temperatures and usually in an interval between 100ºC and 150ºC. However, to obtain a memory cell according to The invention with the desired functional properties may be it is necessary to subject the cell to a conditioning before use before of your actual application in a memory cell, for example, in a memory device This conditioning before use is carried carried out by applying a pulse train of positive and negative voltage alternate to the electrodes of the memory cell for a certain number of impulses or cycles. The applied voltages will be able of generating an electric field between the electrodes capable of polarize memory material in positive or negative direction depending on the polarity of the voltage pulses. Impulses applied voltage will then cause a succession of investments of the polarization direction in the memory material. In this conditioning before use it is evident that the duration and pulse width must be selected and adjusted so that the desired polarization effect is obtained, that is, they must Take into account synchronization and amplitude. Normally the time needed to make the polarization inversion, is that is, a switching of the entire polarization state of the cell of memory, will be in the order of a few microseconds, for example, 50 microseconds approximately. The memory cell can be undergoing a large number of commutations or investments of polarization in conditioning before use. The number of commutations can exceed 10,000 and possibly be greater, but the selection of parameters for optimal conditioning Before use is subject to heuristics.

Memory cells can be used according to the invention as memory cells in memory devices addressable passive matrix, in which case the material of memory, in a preferred embodiment, will be provided as a continuous sandwich layer between the electrode layers first and second, these electrodes being provided as electrodes in parallel tape type so that the electrodes in the first electrode layer are oriented orthogonal with respect to the electrodes in the second layer, thus forming the electrodes a orthogonal electrode array in which memory cells are will then define in the portions of the memory material between transverse electrodes of the electrode layers. Can be devised similarly a passive array memory device based on the embodiment of fig. 6, which is otherwise subject to Norwegian patent 309,500 and therefore will not be exposed in greater detail in the present specification.

Tests have shown that a cell of memory of the present invention and subjected to conditioning Before use it shows excellent fatigue resistance, while that an adequate choice of parameters for conditioning before use it can also serve to form the memory cell much less prone to footprint and disturbance. But first, and before everything, the memory cell according to the invention has resulted virtually immune to fatigue, thus providing a cell of memory with polarization properties that do not deteriorate in the course of a large number of switching cycles in current use, so they can be raised for a successful application in ferroelectric or electret memory devices.

Claims (12)

1. A memory cell comprising a polymeric memory material (2) with ferroelectric or electret properties and capable of being polarized and showing hysteresis, in which the polymeric memory material (2) is provided in contact with the electrodes first and second (3; 4), characterized in that the polymeric memory material (2) is a mixture of at least a first and a second polymeric material (2a; 2b), said first polymeric material being a ferroelectric or electret polymeric material , and because each electrode is a composite multilayer electrode comprising a first layer (3a; 4a) of highly conductive material and a second layer (3b; 4b) of conductive polymer, the conductive polymer forming a contact layer between the highly conductive material (3rd; 4th) and the memory material (2). 2. A memory cell according to claim 1, characterized in that the first polymeric material (2a) is a copolymer. 3. A memory cell according to claim 1, characterized in that the second polymeric material (2b) is a homopolymer. 4. A memory cell according to claim 1, characterized in that the highly conductive material is a metallic material. 5. A memory cell according to claim 1, characterized in that a barrier layer (3; 4) is provided between the first and second electrode layers. 6. A memory cell according to claim 5, wherein the highly conductive material is a metallic material, characterized in that the barrier layer (3c; 4c) is an oxide, nitride or boride of said metallic material. 7. A memory cell according to claim 1, characterized in that the memory material (2) is provided in sandwich between the electrodes. A memory cell according to claim 1, characterized in that the electrodes (3, 4) are provided on opposite surfaces of a bridge (5) of insulating material, the first electrode (3) extending beyond the insulating bridge (5) , and because the memory material (2) is provided on exposed surfaces of the first and second electrodes (3; 4) and extending between the electrodes on the side surfaces of the insulating bridge (5). A memory cell according to claim 1, characterized in that the memory material (2) and the electrodes (3; 4) are provided as thin film layers, so that the memory cell constitutes a thin film structure in layers. 10. A method for conditioning before use of a memory cell (1) comprising a polymeric memory material (2) with ferroelectric or electret properties and capable of being polarized and showing hysteresis and in which the polymeric memory material it is provided in contact with the first and second electrodes (3; 4), and wherein the polymeric material (2) is a mixture of at least one first and a second polymeric material (2a; 2b), said first polymeric material being (2a) a ferroelectric or electret polymeric material, the procedure being characterized by the application to the electrodes of a voltage pulse train of alternating positive and negative pulses that generate an electric field capable of polarizing the memory material in any direction, and subjecting the memory material to a series of successive inversions of the polarization direction thereof. 11. The use of a memory cell according to the claim 1 in a ferroelectric memory device or of addressable passive matrix electret. 12. The use of the memory cell according to the claim 1 in a ferroelectric memory device or of addressable active matrix electret.